Ch. 4: Isomers

Question 1

What is stereochemistry?

Answer 1

Stereochemistry is the field of chemistry that deals with the structures of molecules in three dimensions.

Question 2

Define isomers.

Answer 2

Isomers are compounds with the same molecular formula but different structures.

Question 3

What is the difference between constitutional isomers and stereoisomers?

Answer 3

Constitutional isomers differ in the way their atoms are connected. Stereoisomers differ in the way their atoms are arranged in space.

Question 4

What are the two kinds of stereoisomers?

Answer 4

The two kinds of stereoisomers are cis-trans isomers and isomers that contain asymmetric centers.

Question 5

cis isomer vs. trans isomer

Answer 5

Cis isomer has the substituents on the same side of the ring while trans isomer has the substituents on opposite sides of the ring.

Question 6

chiral vs. achiral

Answer 6

A chiral molecule has a nonsuperimposable mirror image while an achiral molecule has a superimposable mirror image.

Question 7

Define asymmetric center.

Answer 7

An asymmetric center is an atom bonded to 4 different atoms or groups.

Question 8

What are enantiomers?

Answer 8

Enantiomers are nonsuperimposable mirror images.

Question 9

What are diastereomers?

Answer 9

Diastereomers are stereoisomers that are not enantiomers.

Question 10

enantiomers vs. diastereomers

Answer 10

Enantiomers have identical physical and chemical properties. In the case of compounds with 2 asymmetric centers, enantiomers have the opposite configuration at both asymmetric centers.

Diastereomers have different physical and chemical properties. Diastereomers have the same configuration at one asymmetric center and the opposite configuration at the other asymmetric center.

Question 11

achiral reagent vs. chiral reagent

Answer 11

An achiral reagent reacts identically with both enantiomers. A chiral reagent reacts differently with each enantiomer.

Question 12

What is a racemic mixture?

Answer 12

A racemic mixture is one that has exactly 50% of one enantiomer and 50% of another enantiomer.
Bonus: It is indicated by the (+/-) symbol. The individual enantiomers’ rotations cancel out, rendering the racemic mixture optically inactive.

Question 13

What do the letters R and S mean in terms of stereoisomers?

Answer 13

The letters R and S indicate the configuration about an asymmetric center.

R means that the atoms or groups attached to the carbon are arranged from highest priority (1) to lowest priority (4) clockwise.

S means that the atoms or groups attached to the carbon are arranged from highest priority (1) to lowest priority (4) counterclockwise.

Question 14

If one member of a pair of stereoisomers has the R configuration and the other has the S configuration, they are (1) _________.
If they both have the R configuration or both have the S configuration, they are
(2) ________ .

Answer 14

(1) enantiomers

(2) identical

Question 15

Chiral compounds are (1) ___________ ;

achiral compounds are (2) __________ .

Answer 15

(1) optically active

(2) optically inactive

Question 16

If one enantiomer rotates the plane of polarization clockwise (+), its mirror image will rotate it the same amount ___________ .

Answer 16

counterclockwise (-)

Question 17

Each optically active compound has a characteristic ____________ .

Answer 17

specific rotation

Question 18

What is a meso compound? Is it optically active or inactive?

Answer 18

A meso compound has 2 or more asymmetric centers and a plane of symmetry. It is optically inactive.

Question 19

In what circumstance does a compound have 3 stereoisomers? What exactly are those 3 stereoisomers?

Answer 19

When a compound has the same 4 groups bonded to 2 different asymmetric centers, it will have 3 stereoisomers, which are a meso compound and a pair of enantiomers.

Question 20

What atoms other than carbon can have asymmetric centers?

Answer 20

Atoms such as N and P can be asymmetric centers if they are bonded to 4 different atoms or groups.